Electro-optical apparatus

ABSTRACT

An electro-optical apparatus constructed according to various inventive principles disclosed herein provides flexibility in the type and arrangement of components. The electro-optical apparatus preferably includes a Printed Circuit Board (PCB). An interface device can be electrically connected to the PCB, and an electro-optical device can be electrically connected to the interface device. A lens is preferably arranged in optical communication with the electro-optical device. Numerous variations in component selection and arrangement are contemplated within the scope of these inventive principles. Various methods for configuring an electro-optical apparatus are also provided.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to devices for converting betweenelectrical and optical signals. More particularly, this inventionrelates to electro-optical devices and their methods of manufacture anduse.

2. Description of Related Art

Electro-optical modules can be used to convert electrical signals intooptical signals and vice versa. Many different: types of electro-opticalmodules are presently being manufactured. These modules have manyapplications, particularly within data-communications technology whereelectrical signals are carried by fiber optics, and can range in costfrom under a hundred dollars to many thousands of dollars per module,depending on their application and functionality.

Different types of electro-optical modules can be used to performdifferent functions. Receive modules and transmit modules, for example,can each be used to provide half of an electro-optical conversion. Moreparticularly, receive modules convert optical signals into electricalsignals as part of a receive function. Transmit modules convertelectrical signals into optical signals as part of a transmit function.Transceiver modules can be used to perform the electro-opticalconversion for both receive and transmit paths. Transponder modulesprovide the same functionality as transceiver modules but also provideserialization and deserialization of the electrical signals.

Modules can be further categorized based on the type of light emitterused. Typical light emitters include surface emitting sources (such asLight Emitting Diodes (LEDs) and Vertical Cavity Surface Emitting Lasers(VCSELs)) and edge emitting sources (such as Fabry-Perot lasers andDistributed Feedback (DFB) lasers). Surface emitting light sources aregenerally used in the manufacture of low-cost modules.

The primary advantages of surface emitting light sources are in testingand assembly. Surface emitters can, for example, be easily tested onwhole wafers. No assembly of the individual modules is thereforerequired before testing the part. The assembly process is also simplerbecause the edges of the parts do not need to be polished. In addition,surface emitters can be easily assembled into arrays of multipleemitters (for example, a 1×12 array of VCSELs). Surface emitter arraysgreatly simplify the assembly of parallel optical modules.

There are several challenges, however, in manufacturing optical modules.Among these challenges, it is difficult to align an optical fiber to anactive optical area of a light emitter or a light detector. In addition,emitters may degrade or malfunction at relatively low temperatures, andremoving heat from the emitters can be difficult. It is also difficultto test conventional modules. Another challenge is minimizing the numberof parts required in the module assembly. A lack of “batch”manufacturing process steps also prevents lower cost manufacturing ofmodules. Some conventional solutions to these challenges are describedbriefly below.

Manufacturers of optical modules generally use some type of lens system(such as a spherical lens) to focus light into and out of the opticalfiber. FIG. 1 is a schematic illustration of a conventional lensfocusing system for an optical module. Referring to FIG. 1, thisconventional focusing system includes a spherical lens 1, a multi-modeoptical fiber 2 with an optical core 2 a, and an electro-opticalcomponent 3 with active area 3 a. These parts collectively form a systemhaving an optical axis 4 and a ray trace 5. The surrounding material 6is typically air.

The use of a lens is advantageous for at least two reasons. First, itacts as a light gathering element to collect the light from the emittingside. Second, it acts as a light focusing element to converge the lighton the receiving side. These two actions result in a relaxation of thealignment tolerances between the emitting and receiving sides. Ineffect, the lens acts as a large target area for the emitter, whencompared to the size of the receiver, while creating a focused spot thatis small compared to the size of the receiver. Thus, the emitter maymove around to some extent and still hit the lens, and the receiver mayalso move around to some extent and still have the focused spot fallwithin its active area.

Even with a lens, however, the alignment tolerances in a typical modulerequire an active alignment process, which is conducted during theassembly of the optical system. In this process, the optical emitter isswitched on and an output of the optical receivers is measured. Thewhole assembly is then micro-manipulated, typically by a human operator,to maximize the received signal by bringing all sub-components into finealignment. A flash cure process is then typically performed to freezethe assembly in place once fine alignment has been achieved.

There are disadvantages with the current methods for optical alignment.Optical elements such as glass lens arrays may be expensive. They arealso typically small and may be delicate and difficult to handle andmanipulate. Given that the optical assembly must be adjusted for finealignment, some allowance must be made in the module design tofacilitate this adjustment step. Active alignment is a slow, humandriven process and is consequently expensive and error prone. Problemswith optical alignment or failure to align correctly can create asignificant rate of failure during the assembly process, furtherincreasing the production costs of the module.

It would be advantageous to achieve optical alignment without an activealignment step through the inherent construction of the module (called“passive alignment”). It would be further advantageous if the opticalcomponents were very low cost and if there was no requirement to handlethem as a separate subassembly.

Thermal management is also difficult in conventional optical modules.Optical transceiver or transponder modules, for example, typicallyrequire four different types of discrete semiconductor chips in closephysical association with the optical axis. The light emitter and lightdetector are arranged on the optical axis. A driver chip for the emitterand an amplifier chip for the detector are also typically required.Emitters usually consume a significant amount of electrical power. Theconnection between the emitter and the driver chip is a major source ofElectro-Magnetic Interference (EMI) and the signal degrades as thelength of the connection increases. It is therefore advantageous tolocate the emitter driver close to the emitter to limit the length ofthe connection. It is also advantageous to locate the receiver amplifierclose to the detector because the detector output signal is very weakand therefore quickly degrades as the connection length increases.

These four semiconductor chips are therefore typically located in a verysmall area that is physically close to the emitter. Each chip consumespower that is dissipated as heat. In the case of a VCSEL emitter, forexample, this heat may cause the VCSEL to function poorly in terms ofits optical power output (slope efficiency), threshold current, andcenter wavelength accuracy. Or it may cause the VCSEL emitter to stopfunctioning completely. In addition, it may cause premature aging andearly failure of the VCSEL. It is therefore desirable to provide anefficient conduction path for drawing heat away from the VCSEL.

FIGS. 2A and 2B show typical thermal control solutions in a conventionaloptical module. More particularly, FIG. 2A illustrates foursemiconductor devices 7 bonded with die attach material 8 to a circuitboard 9. The circuit board 9 contains thermal vias 9 a and is abutted toa metal heatsink 10. FIG. 2B illustrates two semiconductor components 11bonded directly to a metal heatsink 13 with die attach material 12. Inboth of these examples, the metal heatsink is able to fairly efficientlytransfer heat away from the semiconductor components to the coolersurrounding medium.

Conventional thermal management solutions, however, require that thesemiconductor devices be bonded to thermally conductive materials. Thismay be a disadvantage if it prevents or impacts the ability to opticallyalign or test the module. For example, bonding small optical componentsdirectly to a large heatsink requires manipulation of the entireheatsink in order to accurately align optical components.

In some cases, a separate, small circuit board may be used as an“optical substrate” on which optical die are mounted. The use of anoptical substrate adds additional cost to the system, however, becauseit requires the formation of thermal vias through the substrate andpossibly requires other thermal management. In that solution, a secondcircuit board, without additional thermal management features, is thenrequired to route the signals from the optical substrate to the moduleoutput connector. This arrangement adds complexity, difficulty, and costto the manufacturing process.

It would be advantageous if thermal management of the module could beaccomplished without impacting the alignment of the module, while stillproviding the most optimal path for heat transfer away from the VCSEL.It would be further advantageous if a single circuit board could be usedfor the module to reduce complexity and cost.

Given the complexity and number of sub-components found in a typicalmodule, some form of testing is important to guarantee a sufficientlevel of quality to the end user. Generally, every function of themodule is tested. For example, the operation and performance parametersof the transmit function should be checked. These parameters includelaser output power, extinction ratio, at-speed operation, and jitterperformance. In many cases the drive parameters for each laser in themodule must be electrically adjusted to enable the module to meet itsspecifications over the full operating temperature range. The receiving(RX) channels are tested in a similar manner to verify performance tothe required Bit Error Rate (BER). The module as an entire system shouldalso be tested because the lenses and thermal performance alsocontribute to its overall performance. Module manufacturers thereforetypically only test fully assembled modules. This test can take a longtime (e.g., 10-20 minutes) to conduct and frequently requires a humanoperator to handle the module and conduct the test.

Currently, very few VCSEL suppliers are capable of guaranteeing zerodefects for early life failures for unpackaged parts. Those that canmake this guarantee charge a premium price for their product. Thus,module manufacturers who use VCSELs must burn-in their fully assembledmodules for some period to eliminate modules whose VCSELs fail early intheir life.

One of the fundamental disadvantages of conventional module designs isthat you may only test and burn-in the fully assembled module. Thismeans that defects that may occur at any point in the process are notdetected until the final step. It further means then when a modulefails, the full cost of the assembled module is lost. In addition, themodules are relatively large and difficult to handle with automatedequipment and human intervention is therefore frequently required toinsert and remove these modules from test fixtures.

It would be advantageous if the electro-optical components could befully tested for alignment, operation, and performance as a singlesub-assembly prior to final assembly into a finished module. Thus, thefull cost of the module would not be incurred if the electro-opticalsub-assembly failed. It would be further advantageous if the fullytestable electro-optical sub-assembly were easy to handle and test withautomated equipment. It would be advantageous if the electro-opticalsub-assembly were compact and easy to store in standard sized componenttrays, which are used by most available semiconductor handlingequipment. It would be advantageous if fully tested electro-opticalsub-assemblies were able to be stored in inventory so that modules couldbe quickly assembled with various different styles of heatsink, which isa typical requirement of module customers.

There are also disadvantages with the current methods for manufacturing,as well as with the quality and reliability of the finished product.Conventional modules are generally manufactured manually. They alsotypically include multiple small circuit boards and various othermechanical sub-components that must each be handled and placed in themodule. There are no common form-factor methods (such as industrystandard component trays that exist for semiconductors) for storing orhandling the various sub-components of the optical modules. There istherefore little or no automated equipment available off the shelf toautomate the assembly process. Because the process is primarily manual,it is prone to human error as well as poor process control. This impactsthe manufacturing yield, as well as the quality and reliability of theproduct.

It would therefore be advantageous to have a module design that can bemanufactured using a fully automated assembly process. It would befurther advantageous if this assembly process could be performed usingcommonly available equipment, such as manufacturing equipment used inthe semiconductor industry. It would also be advantageous if theassembly processes and materials used in the module assembly werecommonly used and well understood. A module designed to have theseadvantages would be easier to manufacture at lower cost and at superiorlevels of yield, quality, and reliability.

SUMMARY OF THE INVENTION

An electro-optical apparatus constructed according to various inventiveprinciples disclosed herein can include a Printed Circuit Board (PCB).An interface device can be electrically connected to the PCB, and anelectro-optical device can be electrically connected to the interfacedevice. A lens is preferably arranged in optical communication with theelectro-optical device.

This electro-optical apparatus provides flexibility in the type andarrangement of components. The lens, for instance, can be a molded lensarray having a lens element arranged in optical communication with anactive area of the electro-optical device. Other types of lenses couldalso be used.

Also, a leadframe can be mounted to the PCB beneath a leadframe cutoutregion. The interface device can be mounted to the leadframe using a dieattach material. The interface device can be wirebonded to both the PCBand to the electro-optical device. The interface device could also,however, be flip-chip mounted directly to the PCB and electricallyconnected to the PCB through solder bumps.

The electro-optical device can also be mounted either on the leadframeor the PCB, for example. In one embodiment, the electro-optical deviceis mounted to the leadframe. In another embodiment, however, theelectro-optical device is a flip-chip device arranged on a backside ofthe PCB. In this embodiment, the electro-optical device can beconfigured to optically communicate with the lens through a cutoutregion in the PCB. The electro-optical device can be electricallyconnected to the PCB through wire-bonds, solder bumps, or otherappropriate connection.

The electro-optical device could also be physically mounted on theinterface device, such as using a die attach material. The die attachmaterial could be either conductive or non-conductive. In thisembodiment, the electro-optical device can be wire-bonded or tapeautomated bonded to the PCB, for instance. Numerous other variations incomponent selection and arrangement are also contemplated within thescope of these inventive principles. Various methods for configuring anelectro-optical apparatus are also provided.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and additional aspects and advantages of the presentinventive concepts will become more readily apparent through thefollowing detailed description of preferred embodiments, made withreference to the attached drawings, in which:

FIG. 1 is a schematic illustration of a conventional lens focusingsystem for an optical module;

FIGS. 2A and 2B show typical thermal control solutions in conventionaloptical modules;

FIGS. 3A through 3C are schematic plan and cross-sectional views of anElectro-Optical Sub-Assembly (EOSA) according to an embodiment of thepresent inventive concepts;

FIG. 4A is an enlarged cross-sectional view of the leadframe area of theEOSA shown in FIGS. 3A through 3C;

FIGS. 4B through 4E are enlarged cross-sectional views of a leadframearea of an EOSA constructed according to various alternativeembodiments;

FIG. 5 is a schematic plan view of a leadframe, showing alignmentfeatures thereof, according to yet another inventive concept;

FIGS. 6A through 6D are schematic plan, bottom, and cross-sectionalviews of an Over-Molded Lens Array (OMLA) mold according to a stillfurther inventive concept;

FIG. 7 is a schematic partial cross-sectional view of aflip-chip-mounted semiconductor device and a leadframe mountedsemiconductor device from the leadframe area of FIG. 4B, thermallycoupled to a heatsink;

FIG. 8 shows a standard matrix leadframe used to construct the EOSA ofFIGS. 3A through 3C;

FIG. 9 shows the leadframe of FIG. 8 after completion of the die attachprocess;

FIG. 10 shows a flex PCB panel used to construct the EOSA of FIGS. 3Athrough 3C;

FIG. 11 shows the flex PCB panel of FIG. 10 after mounting of variouscomponents thereto;

FIG. 12 shows a plurality of EOSAs after the flex PCB panel of FIG. 11has been attached on top of the leadframe of FIG. 9;

FIG. 13 shows a plurality of completed EOSAs after wire-bond andover-mold steps are applied to the EOSAs in FIG. 12;

FIGS. 14A and 14B are schematic cross-sectional views of a test fixturefor testing the EOSAs of FIGS. 3A through 3C, constructed according tostill further inventive concepts; and

FIG. 15 is a cross-sectional view of an assembled electro-optical moduleincluding the EOSA of FIGS. 3A through 3C according to yet otherinventive concepts.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present inventive principles will be described more fullyhereinafter with reference to preferred embodiments thereof. It shouldbe noted, however, that the embodiments disclosed herein are provided byway of example, and not by way of limitation, and may be modified inform and detail without departing from the principles and scope of theinventive concepts.

A low cost optical module and a process for manufacturing the opticalmodule are disclosed. According to a preferred embodiment, a paralleloptical module uses surface emitting light sources (such as LEDs andVCSELs) and includes many fibers (e.g., 24 fibers) in a fiber-ribboninterface. The inventive principles herein can also be applied, however,to modules having other light sources and to those having single or dualfiber connections of individual fibers. The inventive principles hereinfurther apply to transceivers, transponders, receive and transmit, andother types of optical modules.

According to the inventive concepts disclosed herein, an Electro-OpticalSub-Assembly (EOSA) can be manufactured using standard semiconductorassembly equipment. Manufacturing processes for the EOSA can thereforebe completed at lower cost and with a shorter manufacturing cycletime,while providing a higher yield and a more reliable and higher qualityproduct as compared to conventional modules. The process also providesthe ability to manufacture a large number of module sub-assemblies inparallel (e.g., twenty or more on a single leadframe). This parallelmanufacturing capability also greatly reduces costs compared toconventional, one-at-a-time manufacturing methods.

An EOSA constructed according to a preferred embodiment can be a fullyintegrated, independently-testable sub-assembly. Thus, test and burn-inprocesses may be performed on this sub-component before it is fullyassembled into an optical module. The ability to independently test thissub-assembly reduces cost, increases the yield of final assembledmodules, and allows greater flexibility by permitting the assembly ofmultiple different module form factors using the same EOSAsub-component.

Electro-Optical Sub Assembly (EOSA)

An optical module according to one embodiment preferably includes asingle, fully testable Electro-Optical Sub-Assembly (EOSA). The EOSA canbe configured, for example, for use in a twelve-channel transceivermodule (i.e., twelve receive and twelve transmit channels). FIG. 3A is aplan view of an EOSA 10 for a twelve-channel transceiver moduleconstructed according to various inventive principles disclosed herein.

Referring to FIG. 3A, the EOSA 10 can be constructed using a flexibleprinted circuit board (flex PCB) 14 that provides interconnections forelectrical signals. A connector array 16 (e.g., a “Berg” or “MegArray”connector) can be soldered to the flex PCB 14 to facilitate electricalcommunication with external circuits. The array of connectors 16 can,for instance, contain an array of either male pins 16 a or femalesockets (not shown). A left end of the flex PCB 14 is preferably coveredby an overmolded lens array (OMLA) 15, formed of anoptically-transparent, injection-molded material. The OMLA 15 in thisparticular embodiment is a 2×12 array of lenses 15 a. These lenses 15 aprovide optical alignment for electro-optical semiconductor componentsarranged beneath the OMLA 15.

FIG. 3B is a plan view of the EOSA 10 of FIG. 3A, shown having the OMLA15 removed to reveal structures arranged thereunder. Referring to FIG.3B, the flex PCB 14 can include a cutout region 14 a. Althoughrectangular in the figures, the cutout region 14 a can be any desiredsize or shape. The cutout region 14 a is preferably arranged to exposean upper surface of a leadframe 18 bonded to an underside of the PCB 14.In this embodiment, two electro-optical semiconductor devices 19 areincluded on the leadframe 18. Each electro-optical semiconductor device19 can include a plurality of active elements. For example, each device19 can include an array of twelve electro-optical elements. Interfacesemiconductor devices, such as a driver chip 20 associated with anemitter and an amplifier chip 25 associated with a sensor, are alsopreferably included.

Referring now to FIGS. 3A and 3B, alignment holes 17 a, 17 b arepreferably arranged through the sub-assembly, including the OMLA 15 andthe leadframe 18, to provide an alignment mechanism for aligning thelenses 15 a of the OMLA 15 over the electro-optical semiconductordevices 19. This alignment mechanism facilitates quick and accuratealignment of the lenses 15 a over active areas of the electro-opticaldevices 19. Any other mechanism that can provide rapid and accuratealignment of the lenses 15 a over the electro-optical devices 19 couldalso be used.

FIG. 3C is a cross-sectional view of the EOSA taken through line 3C—3Cof FIG. 3A. Referring to FIG. 3C, the array connector 16 is preferablyfixed to the flex PCB 14 using solder joints 16 b. The leadframe 18 ispreferably bonded to the back of the flex PCB 14 at the left end of theflex PCB 14. In this embodiment, the electro-optical semiconductordevices 19 and their associated interface semiconductor devices 20, 25are bonded to the surface of the leadframe 18. Die to die wirebonds 22can be used to connect the electro-optical components to theirrespective interface chips 20, 25. Die to flex PCB wirebonds 21 can beused to connect the interface chips 20, 25 to the flex PCB. The OMLA 15preferably covers the entire die and wirebond area and includes aplurality of lens elements 15 a that are optically aligned over activeareas of the electro-optical semiconductor devices 19.

One significant advantage of the EOSA described above is that itincorporates both the optical and electrical functionality of anelectro-optical module into an easily manufactured and testedsub-assembly. Several other advantages and further inventive conceptswill become apparent through the descriptions contained in the followingsections.

Platform Flexibility

The combination of a rigid leadframe with a flexible PCB creates aplatform that can be easily and readily modified to accommodate numerousvariations in the type and arrangement of components used to create thesub-assembly. In the market today, a large number of different devicesare available from various manufacturers that are capable of performingthe electro-optical conversion and interface functions. These devicesvary in size, power dissipation, and in the type of electrical andoptical interfaces. For example, photo-detectors may be supplied withwirebond pads or with solder-bumps to facilitate the electricalinterface. A photo-detector may have two electrical contacts (e.g.,anode and cathode terminals of a diode) on its top side or it may have asingle top-side contact and a single bottom-side contact.

Accordingly, a module technology that permits variations in the type andarrangement of electrical and optical components without significantmodifications is extremely beneficial. The technology described hereinpermits the choice of optimal electro-optical components to provide thedesired functionality of the module. It also provides a flexibleplatform that can readily incorporate improved technologies. Forexample, with this platform arrangement, electro-optical components thatutilize wirebond interconnections may be used for lower-speed modules.For higher-speed modules that require faster components, this platformprovides the adaptability to use solder-bumped components, which operatebetter at higher speeds.

FIG. 4A is an enlarged cross-sectional view of a leadframe area of theEOSA 10 shown in FIGS. 3A through 3C, illustrating in detail anattachment between the semiconductor devices 19, 20, 25 and theleadframe 18 using a die-attach material 23. The die-attach material 23may be electrically conductive or non-conductive. Not all of thesemiconductor chips 19, 20, 25 need to use the same type of die-attachmaterial 23. For example, some may use a conductive die-attach materialwhile others may use a non-conductive die-attach material. The leadframe18 is also preferably bonded to the flex PCB 14 using a bonding material24. The bonding material 24 can, but need not, be the same as thedie-attach material 23 used to attach the semiconductor devices 19, 20,25 to the leadframe 18.

The flex PCB 14 can include via structures 14 b that provide electricalcontact from the flex PCB 14 to the leadframe 18 through a conductiveattach material. These vias 14 b are in addition to any other vias (notshown) within the flex PCB 14 that provide simple interconnectionsbetween the various conductive layers. The OMLA 15 preferably includesintegrated lens elements 15 a. The optical axes from the active areas ofthe electro-optical semiconductor devices through the OMLA lenses 15 aare indicated by dashed lines 31. In FIGS. 4B through 4E, the OMLA 15has been omitted from the drawings for clarity, but can be provided in aform similar to that shown in FIG. 4A.

FIGS. 4B through 4E are enlarged cross-sectional views of the leadframearea of an EOSA constructed according to various alternative embodimentsof the inventive principles herein disclosed. Labels used in thedrawings, and descriptions thereof, apply to similar structures in eachof other FIGS. 4A through 4E, although omitted from some of the drawingsfor clarity. It should be noted that FIGS. 4A through 4E are exemplaryonly and do not reflect every possible embodiment. Rather, these figuressimply illustrate a few of the many possible embodiments enabled by theinventive principles disclosed herein.

FIG. 4B, for example, illustrates an EOSA 10 a in which a flip-chipdevice 20 a provides one of the interface semiconductor devices. Theflip-chip device 20 a has an electrical interface with the flex PCB 14provided through solder bumps 26 rather than wirebonds. A cutout 18 a inthe leadframe 18 accommodates the flip-chip mounting of the interfacesemiconductor chip 20 a to the backside of the flex PCB 14. The cutout18 a can be filled with a thermally conductive material (see FIG. 7) tofacilitate the transfer of heat away from the backside of the device 20a.

FIG. 4C shows an embodiment of an EOSA 10 b in which both theelectro-optical component 19 a and its associated interface component 20a are flip-chip devices, whose electrical interfaces are providedthrough solder bumps 28, 26, respectively, rather than throughwirebonds. In this embodiment, a larger cutout 18 b is provided in theleadframe 18 to accommodate the flip-chip devices 19 a, 20 a. Inaddition, a small cutout 14 c in the flex PCB 14 allows light to passthrough to the optical component 19 a to provide optical communicationwith the OMLA 15.

FIG. 4D illustrates an embodiment of an EOSA 10 c in which a wire-bondedelectro-optical chip 19 b is stacked on top of an associated interfacecomponent 20 b. The die-attach material 29 may or may not be used tocreate an electrical connection between the two devices 19 b, 20 b. Thedie-attach material can, for example, be a silver epoxy (a typicalconductive die-attach material), a solder joint, or any other conductiveor non-conductive material.

FIG. 4E shows another embodiment of an EOSA 10 d similar to the EOSA 10c shown in FIG. 4D. In the embodiment shown in FIG. 4E, however, theelectro-optical component 19 c is connected to the flex PCB 14 usingTape Automated Bonding (TAB) rather than wirebonding. TAB connections 30form an electrically superior interface compared to standard wirebondconnections 22.

Optical Alignment

Another advantage of the Electro-Optical Sub-Assembly (EOSA) constructedaccording to various inventive principles described herein is providedthrough optical alignment of the sub-assembly components. In particular,three elements should be optically aligned to provide functionality tothe electro-optical module. These three elements include a fiber core,the lens elements, and active areas of the electro-optical semiconductordevices.

In a preferred embodiment, alignment pins and associated alignment holesin the EOSA provide an alignment mechanism for locating a fiber ferrulewith respect to the EOSA. Referring to FIGS. 3A through 3C, for example,respectively arranging alignment holes 17 b, 17 a in a rigid leadframe18 and a precision molded lens array 15, helps provide the desiredalignment as an inherent part of the manufacturing process.

FIG. 5 is a schematic plan view of a leadframe 18, showing alignmentfeatures thereof according to a preferred embodiment. Referring to FIG.5, the leadframe 18 preferably includes two precision drilled datumholes 17 b. The leadframe 18 also preferably includes fine alignmentfiducials 32 marked on a surface thereof. The fiducial marks 32 aresmall relative to the size of the datum holes 17 b. Alignment bars 33are further provided to indicate where the electro-optical semiconductordevices should be placed. These features facilitate very accurateplacement of the electro-optical semiconductor devices. Using a visionsystem, for example, currently available die attach equipment is capableof a die placement accuracy of approximately ±5 μm in production.Currently available CNC machining equipment is capable of drilling holesand marking fiducials to an accuracy of about ±5 μm. It is thereforepossible to align the electro-optical semiconductor devices to the datumholes with sufficiently high accuracy for the present application.

Preferred characteristics of the leadframe include rigidity, mechanicalstability, and the capability of forming datum holes and fiducials todesired tolerances. It is also desirable for the coefficient of thermal(CTE) expansion of the leadframe to be similar to the CTE of theelectro-optical devices. Any material that meets these requirements issuitable for manufacturing the leadframe. Accordingly, metals (such ascopper or aluminum), silicon wafers, or printed circuit board substrates(such as Ceramic, LTCC, FR4, or BT resin), or similar materials are allconsidered candidates for the leadframe material.

An Over-Molded Lens Array (OMLA) is also a useful component in providingoptical alignment of an EOSA. The OMLA is preferably formed by injectionmolding a transparent plastic material on top of and around theleadframe. The OMLA mold is preferably designed to create an array oflens elements that are closely aligned with active areas of theelectro-optical semiconductor devices.

FIGS. 6A through 6D are schematic views of an OMLA mold according to apreferred embodiment thereof. FIG. 6A is a plan view of the top plate 34of the OMLA mold. Although FIG. 6A shows a single mold cavity 34 a, itshould be noted that the top plate 34 preferably includes many separatemold cavities for molding multiple parts in parallel. A corner of themold cavity 34 a preferably includes a run-in channel (gate) 38 throughwhich mold compound flows during a molding process. A raised plinth 36in the center of the mold cavity can be used to form an indentation inthe OMLA that contains the lens array. An array of precision depressions37 in the raised plinth 36 can be used to form the lens array.

FIG. 6B shows a cross-section of the mold top plate 34 taken through acenter of the raised plinth 36 in FIG. 6A. FIG. 6C shows the bottomplate 39 of the mold, which has two alignment pins 40. The alignmentpins 40 serve to align the leadframe with the mold top plate 34 duringthe molding operation. FIG. 6D is a cross-sectional view of the moldbottom plate 39 taken through a center of one of the alignment pins inFIG. 6C.

Referring to FIGS. 6A through 6D, two holes 35 are preferably arrangedin the mold top plate 34 to accommodate respective pins 40 from the moldbottom plate 39 and to thereby accurately position the top mold plate 34during the molding process. The pins 40 are also used to accuratelyposition the leadframe in the mold. Fine alignment between the array oflenses in the OMLA and the corresponding electro-optical components onthe leadframe is accomplished, in part, through the accuracy of the moldmanufacture. Specifically, accurate positioning between the alignmentholes 35 and the lens forming depressions 37, helps to ensure accurateoptical alignment between module components after the mold is formed.

Thermal Performance

Improved thermal performance is achieved by directly attaching the heatgenerating components to the heatsink. According to preferredembodiments of the EOSA, excellent thermal performance is providedbecause the leadframe acts as a first part of the heatsink. FIG. 7 is apartial cross-sectional view of the EOSA 10 a in FIG. 4B having aflip-chip-mounted interface semiconductor device 20 a and a leadframemounted electro-optical semiconductor device 19, each thermally coupledto a heatsink 43. FIG. 7 illustrates the excellent thermal pathsprovided according to various principles of the present invention.

More particularly, as shown in FIG. 7, the EOSA 10 a can include anelectro-optical semiconductor device 19 mounted directly onto theleadframe 18. An interface semiconductor device 20 a can be flip-chipmounted to the flex PCB 14. In this embodiment, this entire sub-assemblyis attached to a heatsink 43. Space between the back of the leadframe 18and the heatsink 43, as well as the cavity 18 a containing the flip-chipdevice 20 a, is filled with a thermally conductive material 41. Heatgenerated by the semiconductor devices 19, 20 a may therefore readilytravel to the heatsink 43 through the heat transfer paths indicated bythe arrows 42 a, 42 b.

Manufacturing Method

Another advantage of the EOSA embodiments described above is theirability to benefit from parallel manufacturing processes such as thoseused in conventional semiconductor manufacturing. This provides thefurther ability to leverage existing investments in semiconductorassembly equipment in the EOSA production process. It also permitshigher standards of quality and reliability than can be achieved usinghighly specialized or manual assembly processes. The following sectionsexplain in greater detail a preferred method of manufacturing EOSAs.

Leadframe Starting Material

FIG. 8 shows a matrix leadframe 44 used to construct an EOSA 10according to the preferred embodiment shown in FIGS. 3A through 3C.Referring to FIG. 8, the leadframe 44 preferably contains twentydie-attach paddles 44 a organized in a 2×10 array. Each paddle 44 apreferably includes two precision holes 17 b. Additional toolingalignment holes 45 can also be provided and used to position and movethe leadframe 44 in various pieces of automated handling equipment. Theleadframe 44 can, for example, be formed of a copper alloy material(e.g., alloy 194) and is preferably approximately 0.2 mm thick. Theleadframe 44 can be manufactured using processes from the semiconductorindustry, which have not been previously applied to the production ofelectro-optical modules.

Tolerances for the leadframe dimensions can be ±0.05 mm. The precisionalignment holes 17 b can be created in a separate process using either aprecision CNC machine or laser drill, for example, and preferably meet atolerance equal to or better than ±0.002 mm. FIG. 5 provides a moredetailed view of a leadframe 18 corresponding to one of the leadframepaddles 44 a of FIG. 8.

Die Attach Process

A die attach process is a process by which semiconductor die are bondedto a leadframe or substrate material. The process preferably consists ofplacing a carefully controlled amount of glue onto the leadframe orsubstrate, accurately placing a die on top of that glue, and then curingthe glue to fix the die in place.

FIG. 9 shows the leadframe 44 after completion of the die attachprocess. Four different dies are shown attached to each leadframe paddle44 a. Referring to FIG. 9, several different dies can be used to providethe interface and electro-optical semiconductor devices. For example, atwelve-channel Trans-Impedance Amplifier die 48, a die containing anarray of twelve photo-detectors 49, a die 50 containing an array oftwelve VCSELs, and/or a twelve-channel laser driver die 51 could beused.

Proper alignment between the electro-optical components 49, 50 and thealignment holes 17 b is important. Proper alignment can be achieved, forexample, by using a vision system on the die attach equipment that iscapable of placing the die with a tolerance equal to or better than+0.005 mm. This tolerance is within the capability of die attachequipment currently available. It should be noted that in thisembodiment, the die on the bottom row of the leadframe paddles 44 a arepreferably placed in a pattern that is rotated 180° compared to the diearranged on the top row of the leadframe paddles 44 a.

Flex PCB Manufacture and Attachment to Leadframe

According to a preferred embodiment, described below with reference toFIGS. 10 through 13, a single panel of flex PCB material can be used tomanufacture multiple EOSAs in parallel. FIG. 10 is a schematic diagramillustrating a flex PCB panel 52 having multiple (e.g., twenty) flex PCBpaddles 52 a. FIG. 11 shows the flex PCB panel 52 of FIG. 10 aftermounting various components thereto. FIG. 12 is a schematic plan view ofthe sub-assemblies after the flex PCB panel 52 of FIG. 11 has beenattached on top of the leadframe 44 of FIG. 9.

Referring first to FIGS. 10 through 12, paddle cutouts 14 a arepreferably positioned in the flex PCB paddles 52 a such that die mountedon corresponding leadframe paddles 44 a can be positioned in the paddlecutout 14 a. Locator holes 54 can be provided around the edge of thepanel 52. The locator holes 54 can be used to fix the flex PCB panel 52into a stiffener ring 59. Alignment holes 55 are located at each end ofthe flex PCB panel 52 and are used to align the flex PCB panel 52 to theleadframe 44. Release slots 56 define the final shape of the flex PCBthat will be left in the EOSA.

Passive components 58 and a connector array 16 are then mounted on a topside of each of the flex PCB paddles 52 a. The flex PCB panel 52 can beassembled using a process similar to processes used in the semiconductorindustry. Solder paste can, for example, be screen printed onto the flexPCB paddles 52 a. Various components can then be placed into the solderpaste before passing the PCB panel 52 through a reflow oven to createthe solder joints. Components may be mounted on both sides of the flexPCB paddles 52 a. For instance, a micro-controller orSerializer/Deserializer (SerDes) may be mounted on a bottom side of aflex PCB 52 a, such as beneath the connector array 16.

A plastic or metal stiffener ring 59 is preferably attached to anoutside edge of the flex PCB panel 52. The ring 59 can include two partsthat snap together through the small locator holes 54 in the edge of theflex PCB panel 52. Additionally, pins on the ring 59 preferably protrudethrough the alignment holes 55. These pins can be used to align flex PCBpaddles 52 a with respective leadframe paddles 44 a. Thus, the stiffenerring 59 can serve to hold the flex PCB panel 52 in a flat orientation aswell as to align the leadframe paddles 44 a with respect to the flex PCBpaddles 52 a.

Epoxy glue is preferably applied either to the bottom side of the flexPCB paddles 52 a or to the top side of the leadframe paddles 44 a. Thestiffener ring alignment pins can then be used to position the flex PCBpaddles 52 a on top of the leadframe paddles 44 a and to hold theleadframe 44 in place. Pressure is then preferably applied from both atop and a bottom of leadframe paddle areas while the sub-assembly ispositioned inside a cure oven to cure the epoxy. A strong bond isthereby created between the flex PCB paddles 52 a and the leadframepaddles 44 a.

Wire-Bond & Over-Mold

A wire-bond process can be used to create electrical connections betweenthe die on the leadframe paddles 44 a and the flex PCB paddles 52 a. Forexample, die to die wire-bonds can connect the electro-opticalcomponents to their associated driver or amplifier component. The driverand amplifier can then be wire-bonded to pads on the flex PCB paddles 52a to complete the electrical connection of the subassembly. As mentionedpreviously, however, the EOSA provides a flexible platform that may mixand match wire-bond and flip chip interconnect and/or other electricalinterconnection technologies within the same overall manufacturingprocess.

Once wire-bonding is completed, the full sub-assembly can receive aplasma clean cycle in preparation for an over-mold process step. Plasmacleaning removes any organic contamination and roughens the surfaces toprovide better adhesion for the mold compound. An array mold with twentymold-cavities (or “pockets”) is then preferably used to create the OMLA15, as described previously. FIG. 13 shows the completed sub-assemblyafter the wire-bonding and over-molding steps. Mold residue (from themold run-in channels) is omitted from this figure for clarity.

EOSA Singulation

The completed sub-assembly is then preferably singulated (divided intosingle EOSA units) using a mechanical punch to cut through the variousholding tabs in both the flex PCB panel 52 and the leadframe panel 44. Alaser could also be used to cut the individual EOSAs from the parallelsub-assembly. The singluated EOSAs are then preferably placed in aplastic component tray, similar to conventional trays used for storingsemiconductor components such as Ball Grid Array packages. Conventionalsemiconductor component trays are typically referred to as “JEDEC” traysbecause they are often designed to conform to standards developed byJEDEC (formerly known as the Joint Electron Device Engineering Council).The singulated EOSAs stored in the trays can be further processed usingautomated equipment designed to work with that particular tray design.FIGS. 3A through 3C illustrate one example of a singulated EOSA.

Testability and Burn-in

In addition to the benefits described above, the characteristics of theEOSA also greatly improve its testability over current module designs.The lens array provides the optical interface for a module and should betested in conjunction with the electro-optical components. The EOSAembodiments described previously provide a single sub-component thatcontains both the electrical and optical components. In addition, theelectrical and optical interfaces can be provided on the same side ofthe EOSA. The EOSA can therefore be tested using an automated testplatform similar to standard semiconductor test platforms.

More specifically, a test socket can be designed to accommodate both theoptical and electrical interface in the same plane. The test socket maybe mounted on a tester load board similar to standard semiconductortester load-boards. An automated pick and place handler (such as usedfor semiconductor devices) can then be used to place the EOSA in andremove the EOSA from the test socket. The entire test process cantherefore be accomplished using a method similar to methods used to testordinary semiconductor packages.

Having simultaneous access to both the optical and electrical interfaceprovides the ability to fully test a module in a single process. Theindividual lasers should be tuned for the specific optical environmentin which they will be placed. A micro-controller device within themodule can be programmed with laser specific parameters that store thetuning parameters generated by the test process. To tune a laser, itselectrical input is preferably modulated while simultaneously measuringits optical output power.

As mentioned previously, the fact that the EOSA includes the lens arrayand that both the optical and electrical interfaces are on the sameside, provides the ability to test the EOSA in a manner similar totesting typical semiconductor devices.

Among other aspects, the OMLA of the preferred EOSA embodiments providesa lens array that can be readily aligned and tested.

FIGS. 14A and 14B are cross sectional views of an EOSA test fixtureaccording to another inventive concept herein. FIG. 14A is an explodedcross sectional view of the test fixture 60 and EOSA 10. FIG. 14B showsthe test fixture 60 in a closed position for performing a test of theEOSA 10.

Referring to FIGS. 14A and 14B, the EOSA 10 is preferably handled by atest handler pick-up arm 62. The pick-up arm 62 picks an EOSA 10 from aJEDEC tray, inserts it into the test fixture, removes it after testing,and then places it into another JEDEC tray. The pick-up arm 62 can holdthe EOSA 10, for example, using a vacuum system. In this embodiment, asin standard pick and place auto-handler pick-up arms for semiconductorsystems, a vacuum force is applied through two holes 63 to hold the EOSA10 firmly against the pick-up arm 62. A significant difference betweenthis system and conventional semiconductor systems, however, is theprovision of two tapered alignment pins 64 in the pick-up arm 62. Thesepins 64 not only align the EOSA 10 with the pick-up arm 62, they furtheralign the EOSA 10 with the fiber-optic MTP (MPO) connector 65. The pins64 are preferably tapered to “self-align” the EOSA 10 so that thepositioning of the pick-up arm does not need to be as accurate. As longas the ends of the pins 64 are directed into the EOSA alignment holes 17b (see FIG. 3B), the EOSA 10 will be centered as the pins 64 areinserted.

The test fixture 60 preferably includes a standard PCB test board 69,which includes a base for the test socket 68. The test socket preferablyincludes an electrical test socket 70 that uses standard pogo-pins 71 toconnect to the EOSA array connector as well as an MTP receptacle 66mounted through the test board 69. A female MTP plug connector (ferrule)65 can be fitted inside the MTP receptacle 66. Together, these twocomponents provide an optical socket 61. The MTP receptacle 66preferably allows the ferrule 65 to float on spring loaded mounts,thereby enabling it to move around slightly within the MTP receptacle66. In this manner, when the handler pick-up arm 62 inserts the EOSA 10into the test fixture, the alignment pins 64 cause the ferrule 65 tomove into correct alignment with an EOSA lens array 15 a.

The ferrule 65 preferably contains an array of optical fibers 67 thatmatch up with the lenses in the EOSA lens array 15 a. The fibers 67optically connect the EOSA 10 to the optical test equipment (or a simpleloop-back) used to test the EOSA 10. The pogo-pins 71 can connect topads on the top side of the test board PCB 69, which are preferably inturn connected to the electrical tester to be used. The MTP receptacle66 and ferrule 65 preferably conform to International ElectrotechnicalCommission (IEC) standard 61754-7.

Current VCSEL diodes require production burn-in to eliminate early lifefailures. In this embodiment, the burn-in process can be performed bymounting the EOSAs onto a burn-in board and then placing the burn-inboard in an oven. The burn-in board can be configured with an array ofsockets that allow several EOSAs to be simultaneously connected throughtheir electrical connectors. These sockets can provide power andstimulus signals for the VCSELs and other components mounted within theEOSA. During burn-in, there is no need to interface to the opticalsocket. By configuring the EOSA in a manner similar to standardsemiconductor parts, the burn-in process and equipment can also besimilar to that used in the semiconductor industry.

Module Assembly

Fully tested and burned-in EOSAs are assembled into modules. FIG. 15 isa schematic cross-sectional view of an assembled module 80. Theassembled module 80 preferably contains three main components, aheatsink 82, an EOSA 10, and an MTP receptacle 81. The heatsink 82 formsthe main body of the module. The heatsink 82 preferably includes twoprecision placed alignment pins 83, which are used to properly align theEOSA 10 and the MTP connector of the fiber ribbon cable. The MTPreceptacle 81 can be arranged in a proximal end of the module 80.

These components can be assembled in any one of several sequences. Inone example, the EOSA 10 is placed into the end of the MTP receptacle81. The heatsink alignment pins 83 are inserted into the back-side ofthe EOSA 10, and the MTP receptacle 81 can then be secured to theheatsink 82 with screws. The EOSA 10 is thereby held in place betweenthe MTP receptacle 81 and the heatsink 82, and is correctly alignedusing the alignment pins 83. The EOSA 10 can then be folded-upunderneath the heatsink 82. The EOSA 10 is preferably secured to thebottom of the heatsink 82 with epoxy glue.

While the inventive principles herein have been shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from those principles. The various alternativeembodiments described below, for example, represent changes to the formand details of the embodiments described previously while retaininginventive principles thereof.

Among other variations, alternatives to the Over-Molded Lens Array(OMLA) are available. For example, instead of molding the lens arraydirectly on top of the leadframe, it is possible to injection-mold aseparate lens array that is then fitted over the leadframe afterwire-bonding. This alternative method provides a slightly lowertechnology risk, because separately molded lens arrays can bemanufactured using widely-available technology.

Unfortunately, however, this method may have slightly higher costsbecause the lens arrays may be molded one at a time in a separateprocess that does not utilize the existing semiconductor mold equipment.Even when molded in parallel, however, the extra step of fixing theindividual lens arrays to the leadframe adds cost and complexity. Thisvariation may also provide slightly lower reliability because thesemiconductor dies may not be completely sealed from the outsideenvironment. In addition, this process may result in a slightly loweryield because the alignment tolerance between the lens array and theelectro-optical die may be less accurate when compared to the OMLA.

Alternative methods for aligning the electro-optical die on theleadframe are also possible. One way to improve the accuracy of thealignment between the electro-optical die and the leadframe is to createa Silicon Optical Alignment Pallet (SOAP). A SOAP is a small rectangleof silicon in which two alignment holes (for the two alignment pins) areetched all the way through, and in which accurate fiducials are markedwith reference to those holes. The SOAP is first attached to theleadframe and the electro-optical die are then placed on top of thepallet and aligned to the fiducials on the silicon. It is expected thatthe SOAP will be more accurate than holes drilled in the leadframe andassociated fiducials marked on the leadframe. This solution is slightlymore costly, however, than the earlier described solution and also addscomplexity to the construction of the EOSA.

Alternative flex PCB configurations are possible. For instance, insteadof a single piece of flex PCB, it is possible to use rigid flex PCB. Inone embodiment using this alternative, the flex PCB has rigid panelslaminated to each end (one over the leadframe end and the other over thearray connector end). By forming accurate holes in the rigid areas ofthe flex PCB, it may be possible to remove the leadframe entirely fromthe construction. This would provide a cost advantage and simplify theconstruction of the EOSA. A disadvantage, however, is that the thermalconduction path from the electro-optical die must then pass through anorganic substrate (i.e., the PCB) with higher thermal resistance, ratherthan through the leadframe. In addition, the alignment holes may not beas accurately placed in the rigid PCB panels as compared to theleadframe, which would impact the final yield of the EOSAs.

Having described the foregoing inventive concepts in terms of severaldifferent embodiments and alternatives, it should be understood thatstill additional variations and configurations are available within thelevel of ordinary skill in the art. The inventive concepts shouldtherefore be interpreted to encompass all such variations coming withinthe spirit and scope of the appended claims.

1. An electro-optical apparatus comprising: a printed circuit (PCB)having at least one cutout region; an interface device electricallyconnected to the PCB; an electro-optical device electrically connectedto the interface device; a lens arranged in optical communication withthe electro-optical device; wherein the interface device is flip-chipmounted on a backside of the PCB and wherein the interface device iselectrically connected to the PCB through solder bumps; and wherein theelectro-optical device is a flip-chip device arranged on a backside ofthe PCB in optical communication with the lens through an optical devicecutout region in the PCB, and wherein the electro-optical device iselectrically connected to the PCB through solder bumps.
 2. The apparatusaccording to claim 1, further comprising a connector array arranged onthe PCB, said connector array configured to communicate electricalsignals with an external device.
 3. The apparatus according to claim 1,wherein the lens comprises a molded lens array having a lens elementarranged to optical communication with an active area of theelectro-optical device.
 4. The apparatus according to claim 1, whereinthe at least one cutout region comprises a leadframe cutout region, andfurther comprising a leadframe mounted to the PCB beneath the leadframecutout region, wherein the interface device is mounted to the leadframeusing a die attach material and wherein the interface device iswirebonded to both the PCB and to the electro-optical device.
 5. Anelectro-optical apparatus comprising: a printed circuit board (PCB)having at least one cutout region; an interface device electricallyconnected to the PCB; an electro-optical device electrically connectedto the interface device; a lens arranged in optical device communicationwith the electro-optical device; wherein the interface device isflip-chip mounted on a backside of the PCB and wherein the interfacedevice is electrically connected to the PCB through solder bumps; andwherein the electro-optical device is physically mounted on theinterface device using a die attach material, and wherein theelectro-optical device is wirebonded to the PCB.
 6. The apparatusaccording to claim 5, wherein the die attach material is conductive. 7.The apparatus according to claim 5, further comprising a connector arrayarranged on the PCB, said connector array configured to communicateelectrical signals with an external device.
 8. The apparatus accordingto claim 5, wherein the lens comprises a molded lens array having a lenselement arranged in optical communication with an active area of theelectro-optical device.
 9. The apparatus according to claim 5, whereinthe at least one cutout region comprises a leadframe cutout region, andfurther comprising a leadframe mounted to the PCB beneath the leadframecutout region, wherein the interface device is mounted to the leadframeusing a die attach material and wherein the interface device iswirebonded to both the PCB and to the electro-optical device.
 10. Anelectro-optical apparatus comprising: a printed circuit board (PCB)having at least one cutout region; an interface device electricallyconnected to the PCB; an electro-optical device electrically connectedto the interface device; a lens arranged in optical communication withthe electro-optical device; wherein the interface device is flip-chipmounted on a backside of the PCB and wherein the interface device iselectrically connected to the PCB through solder bumps; and wherein theelectro-optical device is physically mounted on the interface deviceusing a die attach material, and wherein the electro-optical device istape automated bonded to the PCB.
 11. The apparatus according to claim10, wherein the PCB comprises a flexible printed circuit board having acutout region, wherein the leadframe is a rigid leadframe arrangedbeneath the cutout region of the PCB.
 12. The apparatus according toclaim 11, wherein the electro-optical device is mounted on theleadframe.
 13. The apparatus according to claim 10, wherein theelectro-optical device is flip-chip mounted on a backside of the PCB.14. The apparatus according to claim 10, wherein the electro-opticaldevice is mounted on the interface device.
 15. The apparatus accordingto claim 10, further comprising a connector array arranged on the PCB,said connector array configured to communicate electrical signals withan external device.